1. Field of the Invention
The present invention relates to a magnetic carrier and a two-component developer that are used in an electrophotographic method and an electrostatic recording method.
2. Description of the Related Art
The developing systems used in electrophotography include monocomponent developing systems, which use only toner, and two-component developing systems, which use a mixture of toner with a magnetic carrier. The two-component developers used in two-component developing systems provide a high opportunity for contact between the toner and the magnetic carrier, which is the charge-providing member, and provide stable triboelectric charge characteristics and thus offer the advantage of maintaining a high image quality. In addition, they are used in particular in high-speed machines because the magnetic carrier also feeds a large amount of toner to the development segment and because control is easily performed. The magnetic carrier is typically a resin-coated carrier in which a resin is coated on a magnetic core material, e.g., ferrite, and a variety of proposals have been made with regard to the core material, coating resin, additives, and so forth.
In Japanese Patent Application Laid-open No. 2011-43738, a magnetic carrier is proposed that uses a magnesium ferrite containing a very small amount of Mn for the core material. While having the magnetization and resistance coexist in balance is problematic with a magnesium ferrite, by adding a very small amount of Mn to the magnesium ferrite the structure of the crystal grains is made nonuniform and the crystal boundary is made discontinuous, which brings about a balanced coexistence of the magnetization with the resistance. However, because the crystal grains are made nonuniform, the mechanical strength of the core is then deficient and the carrier particles can undergo cracking and chipping when subjected to stress in the developing device. The thusly produced cracked and chipped particles fly over to the photosensitive member and, under the effect of the pressure applied in the transfer step and cleaning step, these cracked and chipped particles gouge into the photosensitive member and negative effects on the image may then be produced.
In Japanese Patent Application Laid-open No. 2011-164225, a resin-filled magnetic carrier is proposed in which a porous magnesium ferrite containing a very small amount of Sr is used for the core material and a resin is filled and coated in the pores of the ferrite. This resin-filled magnetic carrier then takes the form of a reduced specific gravity carrier that uses a porous magnesium ferrite particle for the core material, and the stress applied to the developer is reduced and a higher quality image can be obtained longer than for a conventional ferrite carrier. In addition, the addition of the Sr results in an inhibition of the variance in magnetization among core particles and also facilitates control of the pore volume in the core. However, the magnesium ferrite constituting the core has a very low resistance, and as a consequence a charge leakage-induced disturbance of the latent image and generation of white spots on the image may occur. While the charge leakage can be avoided by carrying out development at a lower field strength, the force that causes toner flight is then also weakened and an adequate amount of toner may not take flight and the necessary image density may also not be obtained. Moreover, since the silicone resin that is the coating resin is easily sliced off, the resistance of the magnetic carrier can undergo a sharp decline and in particular the toner may not be adequately charged in a high-temperature, high-humidity environment and negative effects on the image, for example, fogging, may also occur. In addition, the admixture of the sliced-off silicone resin into the toner can also cause impaired fixing of the toner and can cause unevenness in the gloss of the image post-fixing.
While methods for improving the stress resistance and stability of two-component developers have been investigated as noted above, the appearance is still desired of a two-component developer that exhibits a very durable stability and that can provide a high-quality image free of image defects on an extended basis.